One very peculiar omission runs through the many surveys that set out to rate "Britain's top employers": how well they provide for the large numbers of parents among their workers. It is as though family life and the qualities of a workplace were entirely separate, rather than being, as they are in the lives of most employees, tightly bound together. At least part of the reason stems from the employers themselves: comparatively few seem to showcase strong parental benefits (if they have them) among their recruitment incentives, or as evidence of high corporate ethics.

Yet any working parent knows how damaging it is to productivity and creativity - not to mention health - to work for an organisation that blanks out, or is actively hostile to, the beating family hearts of its people. And when the wider social fabric is taken into account, how can family policy escape being a key measurement of corporate citizenship? Arguably, giving good support to parents is a social contribution as important as a company's charitable donations or recycling efforts.

These were among the considerations that spurred the Guardian to launch what we believe is the first survey of the parenting benefits on offer from a cross-section of major British employers. What was the survey looking for, and how was it carried out? We decided that in the private sector, the heavyweight foundation of the audit should be Britain's biggest companies and firms, mostly drawn from the top tier of the FT 500, as determined by market valuation or turnover; in some cases we also took workforce size into account. Some of these companies are British-owned; many are British subsidiaries of international corporations.

Among such companies, quite a few are unknown by the public under their corporate names, though the product brands they own may be familiar; so we also aimed to include some household names, even if they did not rank near the top of the FT 500. Charities - often big businesses nowadays - made for another group in the survey. And we invited readers, via the news-paper and the website, to nominate employers they saw as offering good terms for parents, whether in the private or public sector.

In the state sector, we looked for a cross- section of the employment bodies with the biggest workforces or budgets, and added in others to broaden the range of specialised functions. Because Britain's biggest organisations tend to be based in London, we extended into local and regional development authorities - entities that often are the biggest employers in the area. Across private and public sectors, we also aimed to have the United Kingdom fully represented: Northern Ireland, Wales, Scotland, and England. Even so, we had to leave at least one important area for a later date: the primary and secondary school sector, which needs a whole study in itself due to its size and fragmentation, with each school now setting its own employment terms.

So those were the factors that formed our survey group - not an exact science, but one aiming for a common-sense logic. A method driven also by our aim of identifying "best practice", so focusing on the entities most likely to set the benchmarks. But we know this focus leaves at least one important area unexamined: maternity and paternity conditions for the hundreds of thousands of people in Britain who work hard yet are not formally "employees" - ranging from the self-employed to those whose conditions have often been highlighted in Guardian stories on the contract-labour agencies that supply armies of workers for the likes of food-packing plants and hotel and office cleaning. Towards such workers, employer obligations are minimised; and in the contract-labour field, the precarious nature of the work means that even when a new mother earns enough to qualify for state maternity allowance, she will rarely dare to stop work for long.

Once the results began coming in, one immediately striking feature was the tone in which some employers conveyed their maternity and paternity terms to their employees. Breathlessly, the in-house literature from a telecoms company announces that it "provides all pregnant employees with up to 52 weeks' maternity leave regardless of their length of service with the company" - when, in fact, that is a legal requirement. Unpaid parental leave is frequently portrayed as a company perk for fathers when it, too, is the law. Or this from a financial company: "Mothers on maternity leave still have access to a variety of benefits; for example, they remain on the same pay, still have access to free healthcare, they have the ability to accrue their holiday and, if applicable, they can keep their company car." That, too, is the law for any period of paid maternity leave. But until a baby is on the way, most employees have no reason to know these statutory rights. For all the essentials, see our primers.

Once we had marshalled the information from our survey group - more than 250 organisations around the country - how did we rate them? First, we assessed the best in the context of their own category, because standards varied widely between sectors. On the face of it, this approach seems unfair for some employers: a benefits package classed as "poor in sector" in the financial sphere might be well above average in leisure. But it is in its own field that a company or public body is mainly competing for staff and shadowing the policies of rivals, so this seems the most valid arena for the survey's basic verdicts.

Within each sector, employers' parenting packages largely ranked themselves: the best, above average, and poor were the main categories. To be above average or better, an organisation had to offer significantly more than the improved legal minimums in pay and benefits that took effect in April. Those deemed poor were failing to improve appreciably on the legal minimum compared with others in their group.

For our rankings, the make-or-break criteria were good pay during maternity leave (because mothers are likely to be off work far longer than fathers), and how long a company required a person to work before she qualified for that pay. A woman knows she is already jeopardising her future earnings potential by stepping out of the marketplace to have or adopt a child - but being immediately penalised by plummeting income during leave is probably the biggest single disincentive to having a child, and the biggest coercive force in getting her back to work earlier than she might wish. We also judged two years' work about the maximum a woman should need to qualify for decent company maternity pay (some firms ask women to wait two to five years for paltry enhancements over the legal minimum).

There was one further group of employers, mostly from the private sector, that ruled themselves "out of the running" in various ways - either by giving insufficient detail on their parenting benefits, failing to respond to requests for information, or declining to contribute. A few flatly said no. Others said they were "reviewing policy" or in the throes of a merger or takeover; others said they could not respond because "the HR person is away". These all seemed indicative of the company ethos on the subject of maternity and paternity provision. If the package is good, why hide it pending review? And why not shout about it in hopes of seeing it preserved during a merger? If information resides with just one or two HR people, isn't the company's interest in the subject limited? Some also refused to take part because their commercial group covered many and varied subsidiaries - clearly no priority had been given to unifying and upgrading to one standard. By contrast, interested participants in the same boat solved that problem easily, by pulling out the parenting data from one of their subsidiaries with a large workforce.

Finally, navigating through all this, we weighed up the leaders from all sectors to identify the half-dozen best employers in Britain for parents - those emerging from the survey as offering the strongest combinations of well-paid leave, flexible working arrangements and supportive childcare schemes. Their names and other lessons and conclusions from the survey appear at the end of this special report.

But this is, still, a patchwork. We plan to return to the subject of maternity and paternity benefits offered by Britain's employers, and we want to enlist the help of readers, companies, government bodies, and any other organisations to do this. In this Guardian special, we list the best we've found; now tell us the best you know. We'll report back in a future edition.